An RFID tag consists of a microchip attached to a radio antenna mounted on a substrate. The purpose of the tag is to store data which can be altered or retrieved via radio frequency. There are three types of tags: passive tags, semipassive tags and active tags. The passive RFID tags have no battery and require an external source to invoke a signal transmission. Semi-passive tags also require an external source to activate them, but have a significantly higher forward link capability providing a greater read range. An active RFID tag contains a battery and can transmit signals autonomously. The RFID reader interrogates RFID tags that are in the reading range using radio communication.